In humans, luteinizing hormone (LH) and human chorionic gonadotropin (hCG) regulate the secretion of progesterone from the corpus luteum during the menstrual cycle and early pregnancy. This proposal will enhance understanding of how these gonadotrophs regulate luteal cell function by adding to our knowledge of the physical events surrounding hormone interactions with cell surface receptors and other membrane components. Similarities between human and sheep luteal cells make the ewe an excellent model system for studying physical events accompanying hCG or LH binding to the LH receptor. The binding affinity, rate of internalization, and cellular response to brief and prolonged hormone exposure have been characterized for hCG and LH on ovine luteal cells. The goal of this proposal is to examine LH receptor aggregation, distribution, and molecular motions on ovine luteal cells during the luteal phase of the estrous cycle and to explore how the physical behavior of the LH receptor is affected by its interactions with oLH or hCG. First, the extent of LH receptor aggregation will be measured on viable cells before and after hormone binding using fluorescence energy transfer and measurements of LH receptor rotational motions. Receptor distribution will be examined using computer-enhanced low light microscopy and electron microscopy. Second, the role of the alpha and beta subunits of hCG and oLH and the carbohydrates associated with these molecules in modifying lateral and rotational motions of hormone-occupied LH receptors will be determined. The effects of heterodimers formed from hCG and oLH subunits and deglycosylated hCG on the distribution and motion of LH receptor will be examined. Third, our preliminary studies indicate that hCG-LH receptor lateral motions are constrained by cytoskeletal components. We will characterize cytoskeletal interactions with the LH receptor before and after hormone binding. Finally, we will attempt to determine whether other luteal cell membrane proteins are interacting with the hormone-occupied LH receptor. Such interactions may significantly restrict the lateral movement of the LH receptor. Colorado State University has an unusual confluence of expertise in reproductive physiology and of instrumental facilities for studies of membrane protein dynamics. This situation affords us a unique opportunity to obtain new and detailed information regarding membrane events which occur following hormone binding to the LH receptor.